1. Field of the Invention
The invention is related to the field of metallurgy. The invention is related to the field of a direct current electric arc furnace for steel making.
2. Description of the Related Art
In the conventional way of steel making using an electric arc furnace having considerable capacity, an alternating current electric arc furnace is utilized since the supply of electric power and the control of the voltage are easy to handle.
On the other hand the recent progress in the semiconductor ARP enables the capacity of a power supply for direct current to increase, which gives rise to switching from an alternating current electric arc furnace to a direct current electric arc furnace.
The electric facility of this direct current electric arc furnace is the same with that of the alternating current furnace with respect to the power supply including a transformer. In case of the direct current electric arc furnace the electric voltage is lowered by the transformer to the level required by the furnace and alternating current is rectified to direct current by a rectifier such as a thyrister. In such a thyrister system a direct current reactor is utilized to prevent the excessive increase of the elecric current when the direct current system is shortcircuited. The direct current circuit is composed of a primary conductor, or a circuit from the rectifier to an anode and a secondary conductor, or a circuit from a movable carbon electrode to the rectifier.
In the conventional alternating current electric arc furnace the number of electrodes is three. In case of a direct current electric arc furnace number of the electrodes is not necessarily three, and may be at least one. Accordingly in the direct current electric arc furnace a single electrode is possible for the furnace whose capacity is 150 tons per heat, which simplifies the structure of the furnace around the electrode, but which requires a bottom electrode for the bottom of the furnace.
FIG. 5 is a current flow diagram of a direct current electric arc furnace. As shown in FIG. 5 the secondary side terminals of the transformer 1 are connected to the alternating current terminals of the thyrister 2. The minus direct current terminals are connected to the movable electrode 6 of the direct current electric furnace 5 via the direct current reactor 4 and the conductor 3. The plus direct current terminals are connected to the bottom electrode 7 via the conductor 3a. Thus the feeding circuit A-B-C-D-E-F-G-A of the current I is composed. The bottom electrode 7 is situated at the center of the bottom of the furnace 5. The arc 10 forms an electric path between the movable electrode 6 and the steel bath 9. The holder arm 8 holds the movable electrode 6 and moves it up and down. A part of the conductor is flexible and hangs in the vicinity of the furnace 5 forming a U-shaped catenary. In this arrangement the magnetic field of the arc 10 and that of the conductor 3 interfere with each other and the position of the arc cannot be maintained at the center of the furnace 5.
As shown in FIG. 5 the magnetic field B.sub.1 generated by the feeding system, cooperating with the electric current I, exerts a force "f" on the arc which moves the position of the arc from the center. As the result of this deviation the steel making reaction is accelerated at the side in the furnace where the arc is deviated. However at the other side in the furnace the reaction is retarded.
Hence the total time for steel making is increased, the total consumption of the electricity is increased and the wear of the refractory is increased due to the uneven reaction.